Yogurt is generally known to be a product formed from milk which has been heated to an incubation or fermentation temperature, cultured with a yogurt producing culture of, for example, Lactobacillus bulgaricus and Streptococcus thermophilus, and incubated for a period of time sufficient to produce a pH of about 3.5 to about 5.0. The yogurt is then refrigerated to prevent the growth of microorganisms, and packaged for sale. The yogurt product produced by the aforementioned method must be refrigerated until a time just prior to its being eaten to prevent spoilage. Refrigerated, yogurt is stable for no more than a few weeks. The refrigeration requirement presents problems in handling, shipping and marketing.
Yogurt, most commonly marketed as the refrigerated product, has become of great commercial significance. Yogurt products have recently been and are projected to continue to be the dairy product showing the greatest market growth. The increasing popularity of yogurt, and the inherent disadvantages of handling, shipping and marketing a refrigerated, easily spoiled food product has promoted research efforts to produce a shelf-stable yogurt product. A yogurt product which is stable for several months at room temperature would have the distinct advantages of greatly reducing handling and marketing costs.
A number of research efforts have been directed to producing a shelf-stable yogurt product. Most of these efforts have involved heating the yogurt to a temperature which kills or inactivates microorganisms present in yogurt that could cause spoilage. These microorganisms were initially thought to be solely responsible for the instability that ordinary yogurts display upon room temperature storage. Later, it was discovered that milk proteins present in the yogurt also play an important role in the stability of yogurt products. It is presently believed that denaturation syneresis or "wheying off" that occurs during denaturation of milk proteins may be at least partly responsible for this instability.
As proteins denature, they unfold, changing conformation. This unfolding may result in a change in the solubility characteristics of the protein. A protein in solution or suspension before denaturation may precipitate upon denaturation as a result of the change in solubility characteristics. Milk protein, exposed to acidic conditions at room temperature over long periods of time would be expected to undergo denaturation. The change in solubility characteristics of milk proteins in yogurt causes by denaturation may be partially responsible for the limited shelf/life that storage stable yogurts of the prior art have exhibited. Perhaps a second cause of the limited shelf/life of storage stable yogurts is the syneresis phenomenon that milk proteins exhibit during denaturation.
The syneresis phenomenon is actually a two-stage process; a wheying-off stage and a curd forming stage. In the "wheying off" stage certain proteins, for example, beta-lactoglobulin and alpha-lactalbumin separate from the casein protein complex as a viscous liquid which generally appears on the surface of the yogurt. In the curd-formation stage, the remaining protein may complex with calcium or other divalent cations resulting in an insoluble complex, or curd. When the protein in yogurt undergoes syneresis, the curd and whey formation may result in a less desirable yogurt product due to a change in solubility characteristics that result in an unacceptable or gritty texture, and may also contain lumps in the form of curd.
A partial solution to the problem of denaturation and syneresis occurring during storage at room temperature has been the utilization of a heating step which results in partial denaturation of the milk proteins responsible for syneresis. It is believed that partial denaturation of the protein before storage at room temperature minimizes the effects of syneresis occuring during storage at room temperature.
An example of a prior art reference that addresses this problem is Canadian Pat. No. 1,182,682 of Nripen N. Barua and Richard J. Hampton which is incorporated by reference herein. This reference describes a process for treating yogurt to render it shelf stable.
The process of Barua, et al., utilizes a pasteurization step for inactivating spoilage microrganisms and a protein pre-conditioning step for the purpose of denaturing milk proteins. This pre-conditioning or denaturation step is performed under relatively mild conditions. Specifically, the method calls for heating a pasteurized yogurt by one of the two following methods: (1) heating at a temperature from about 140.degree. F. to 160.degree. F. for a period of from 5 to 20 minutes, or (2) raising the temperature of the yogurt without interruption over a period of from about 0.5 to 5.0 minutes from the temperature at which the yogurt was fermented to a homogenization temperature.
The conditions of heating the yogurt in the method of Barua, et al. must be carefully monitored so as to avoid gelatinization of starch, one of the required additives.
Starch has been considered one of the required additives for producing an acceptable shelf stable yogurt product in a number of prior art references. A number of patents in addition to Barua, et al. evidence this fact. (See U.S. Pat. Nos. 3,235,387, 3,932,680 3,969,534 and 4,235,934). However, the use of starch, although required by the methods of these references, is disadvantageous. Because starch tends to gelatinize at high temperatures, the temperature of the protein denaturation step in the method of Barua, et al. must be carefully controlled.
The requirement of the addition of starch as one of the ingredients in the method of Barua, et al. has been found to be unnecessary for producing an aseptic yogurt product. More specifically, the present applicants have found that starch is an unnecessary component in producing a shelf-stable yogurt product, and have further discovered that the addition of starch to a yogurt product which utilizes heat treatment with elevated temperatures to denature protein limits the duration and temperature at which the denaturation step can occur. Furthermore, the use of starch may be disadvantageous during storage at room temperature. It is known that starch is readily cleaved by acid hydrolysis. It is suspected that at the acidic pH of yogurt, the hydrolysis of starch may occur and shorten the duration of storage stability. The hydrolyzed starch may render the yogurt mixture less consistent and more gritty in texture.
It is also believed that all starches in part provide detrimental textural characteristics that affect the organoleptic qualities of the yogurt.
There is therefore a definite need in the art to provide a yogurt product and a process for making such product which does not include starch.